linux-kernel - Re: [PATCH] drm/fbdev-generic: optimize out a redundant assignment clause

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Message-ID: <4e2a2222-59c2-2935-08a7-4a661d5073b2@suse.de>
Date:   Thu, 30 Mar 2023 09:26:32 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Sui Jingfeng <15330273260@....cn>,
        Lucas De Marchi <lucas.demarchi@...el.com>
Cc:     David Airlie <airlied@...ux.ie>, liyi <liyi@...ngson.cn>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/fbdev-generic: optimize out a redundant assignment
 clause

Hi

Am 30.03.23 um 09:17 schrieb Sui Jingfeng:
> Hi,
> 
> On 2023/3/30 14:57, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 30.03.23 um 06:17 schrieb Lucas De Marchi:
>>> On Wed, Mar 29, 2023 at 11:04:17AM +0200, Thomas Zimmermann wrote:
>>>> (cc'ing Lucas)
>>>>
>>>> Hi
>>>>
>>>> Am 25.03.23 um 08:46 schrieb Sui Jingfeng:
>>>>>  The assignment already done in drm_client_buffer_vmap(),
>>>>>  just trival clean, no functional change.
>>>>>
>>>>> Signed-off-by: Sui Jingfeng <15330273260@....cn>
>>>>> ---
>>>>>  drivers/gpu/drm/drm_fbdev_generic.c | 5 ++---
>>>>>  1 file changed, 2 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/drm_fbdev_generic.c 
>>>>> b/drivers/gpu/drm/drm_fbdev_generic.c
>>>>> index 4d6325e91565..1da48e71c7f1 100644
>>>>> --- a/drivers/gpu/drm/drm_fbdev_generic.c
>>>>> +++ b/drivers/gpu/drm/drm_fbdev_generic.c
>>>>> @@ -282,7 +282,7 @@ static int drm_fbdev_damage_blit(struct 
>>>>> drm_fb_helper *fb_helper,
>>>>>                   struct drm_clip_rect *clip)
>>>>>  {
>>>>>      struct drm_client_buffer *buffer = fb_helper->buffer;
>>>>> -    struct iosys_map map, dst;
>>>>> +    struct iosys_map map;
>>>>>      int ret;
>>>>>      /*
>>>>> @@ -302,8 +302,7 @@ static int drm_fbdev_damage_blit(struct 
>>>>> drm_fb_helper *fb_helper,
>>>>>      if (ret)
>>>>>          goto out;
>>>>> -    dst = map;
>>>>> -    drm_fbdev_damage_blit_real(fb_helper, clip, &dst);
>>>>> +    drm_fbdev_damage_blit_real(fb_helper, clip, &map);
>>>>
>>>> I see what you're doing and it's probably correct in this case.
>>>>
>>>> But there's a larger issue with this iosys interfaces. Sometimes the 
>>>> address has to be modified (see calls of iosys_map_incr()). That can 
>>>> prevent incorrect uses of the mapping in other places, especially in 
>>>> unmap code.
>>>
>>> using a initializer for the cases it's needed IMO would make these kind
>>> of problems go away, because then the intent is explicit
>>>
>>>>
>>>> I think it would make sense to consider a separate structure for the 
>>>> I/O location. The buffer as a whole would still be represented by 
>>>> struct iosys_map.  And that new structure, let's call it struct 
>>>> iosys_ptr, would point to an actual location within the buffer's
>>>
>>> sounds fine to me, but I'd have to take a deeper look later (or when
>>> someone writes the patch).  It seems we'd replicate almost the entire
>>> API to just accomodate the 2 structs.  And the different types will lead
>>> to confusion when one or the other should be used
>>
>> I think we can split the current interface onto two categories: 
>> mapping and I/O. The former would use iosys_map and the latter would 
>> use iosys_ptr. And we'd need a helper that turns gets a ptr for a 
>> given map.
>>
>> If I find the tine, I'll probably type up a patch.
>>
>   Here i fix a typo, 'tine' -> 'time'
> 
> As far as i can see, they are two major type of memory in the system.
> 
> System memory or VRAM,  for the gpu with dedicate video ram, VRAM is 
> belong to the IO memory category.
> 
> But there are system choose carveout part of system ram as video 
> ram(i915?,  for example).
> 
> the name iosys_map and iosys_ptr have no difference at the first sight, 
> tell me which one is for mapping system ram
> 
> and which one is for mapping vram?

As you say correctly, graphics buffers and be in various locations. They 
can even move between I/O and system memory. The idea behind iosys_map 
("I/O and/or system mapping") is that it's a single interface that can 
handle both.

Best regards
Thomas

> 
> 
>> Best regards
>> Thomas
>>
>>>
>>> thanks
>>> Lucas De Marchi
>>>
>>>> memory range. A few locations and helpers would need changes, but 
>>>> there are not so many callers that it's an issue.  This would also 
>>>> allow for a few debugging tests that ensure that iosys_ptr always 
>>>> operates within the bounds of an iosys_map.
>>>>
>>>> I've long considered this idea, but there was no pressure to work on 
>>>> it. Maybe now.
>>>>
>>>> Best regards
>>>> Thomas
>>>>
>>>>>      drm_client_buffer_vunmap(buffer);
>>>>
>>>> -- 
>>>> Thomas Zimmermann
>>>> Graphics Driver Developer
>>>> SUSE Software Solutions Germany GmbH
>>>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>>>> (HRB 36809, AG Nürnberg)
>>>> Geschäftsführer: Ivo Totev
>>>
>>>
>>>
>>

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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